Undead

Undead, by definition, refers to an entity that is deceased, but still animate.

Classification

Within Minecraft, it refers to an in-game category of mobs that share the following characteristics:

Are damaged by Instant Health and healed by Instant Damage;
Are not affected by Regeneration and Poison;
Take extra damage from Smite;
Do not take drowning damage (husks and regular zombies turn into regular zombies and drowned, respectively);
Are not targeted by the wither.
Causes an armadillo to hide in their shells.

Some (but not all) undead mobs also possess these characteristics:

Are hostile to the player;
Catch fire under sunlight.

List of mobs

Name	Behavior	Notes	Type	Image
Bogged	Hostile	Bogged are variants of skeletons that spawn only in Swamps and Mangrove Swamps. However, they can also spawn in the Trial Chambers from trial spawners.	Skeleton	File:Bogged.png
Camel Husk	Passive	Camel husks are undead versions of camels. They spawn in the form of camel husk jockies in deserts and do not burn in sunlight.	Zombie	File:Camel Husk.png
Camel Husk Jockey	Hostile	Camel husk jockeies are an undead jockey variant that consists of a husk and a parched riding a Camel Husk. They spawn in deserts and do not burn in sunlight.	Zombie/Skeleton	File:Camel Husk.png
Chicken Jockey	Hostile	Baby zombie or variants riding a chicken.	Zombie
Drowned	Neutral	Drowned are common underwater zombie variants that spawn in oceans and rivers. They can be created if a regular zombie enters water and it fails to keep its head above water for 30 seconds straight.	Zombie
Giant‌^{[JE only]}	Hostile(currently has no AI)	Giants are an unused "giant" variant of zombies that can only be spawned via commands. Although they're deemed hostile and undead, they do not have an AI code of their own and do not burn in sunlight.	Zombie
Husk	Hostile	Husks are a variant of zombies that spawn in deserts and do not burn in sunlight. They can be turned into regular zombies if kept underwater for 30 seconds straight.	Zombie
Parched	Hostile	Parcheds are a variant of skeletons. They spawn in deserts and do not burn in sunlight. They shoot Weakness tipped arrows.	Skeleton
Phantom	Hostile	Phantoms are flying monsters that spawn when the player has not slept 3 or more in-game days.	Phantom
Skeleton	Hostile	Skeletons are common ranged monsters that are equipped with bows. They can be burned by sunlight.	Skeleton
Skeleton Horse	Passive	The skeleton horses are a skeleton variant of the horse.	Skeleton
Skeleton Horseman	Hostile	Skeletons riding skeleton horses. Only spawn when lightning strikes a skeleton horse.	Skeleton
Spider Jockey	Hostile	Skeletons or variants riding spiders or cave spiders.	Skeleton
Stray	Hostile	Strays are variants of skeletons that spawn only in frozen, ice, or snowy biomes. They can be created if a regular skeleton enters powder snow and fails to escape within 22 seconds straight.	Skeleton
Strider Jockey	Neutral	A strider jockey consists of a zombified piglin riding a strider.	Zombie
Wither	Hostile	The wither is a three-headed skeletal boss mob that floats and shoots explosive skulls at players and most mobs, except undead mobs.	Skeleton
Wither Skeleton	Hostile	Wither skeletons are tall variants of skeletons found in nether fortresses. They inflict Wither effect, when attacking the player	Skeleton
Zoglin	Hostile	Zoglins are rare zombified variants of the hoglin created when a hoglin leaves the Nether.	Zombie
Zombie	Hostile	Zombies are common monsters that deal melee damage and attack in groups. They can be burned by sunlight and will turn into drowneds if submerged for 30 seconds straight.	Zombie
Zombie Horse	Passive	A zombie horse is an undead zombified variant of the regular horse. It spawns naturally in plains and savanna biomes.	Zombie
Zombie Horseman	Hostile	A zombie horseman jockey consists of a zombie riding a zombie horse.	Zombie	File:Zombie Horsemen.png
Zombie Nautilus	Neutral	Zombie Nautiluses are undead variant of nautiluses, which can be tamed using a pufferfish, can be equipped with nautilus armor, and can be ridden on using a saddle.	Zombie	File:Zombie Nautilus Breathe Java.gif ‌^{[Java Edition only]} File:Zombie Nautilus Breathe Bedrock.gif ‌^{[Bedrock Edition only]}
Zombie Nautilus Jockey	Hostile	Zombie Nautilus Jockeys consists of a drowned riding a zombie nautilus.	Zombie	File:Zombie Nautilus Jockey.png
Zombie Villager	Hostile	Zombie villagers are zombified variants of the villagers.	Zombie
Zombified Piglin	Neutral	Zombified piglins are zombie variants of piglins and piglin brutes that inhabit the Nether. They were originally called "Zombie Pigmen." Zombified piglins can be created if regular piglins or piglin brutes leave the Nether and fail to return quickly enough.	Zombie

Trivia

Despite the giant being an oversized variant of the zombie, it's not considered an undead mob.
Despite their ghost-like appearance, ghasts are not considered an undead mob.^[1] However, they are never targeted deliberately by the Wither boss.
The Zoglin is the only undead mob that is capable of attacking other undead mobs^[2], and the only one that does not spawn naturally.